Many autoimmune and chronic inflammatory diseases are related to immunoregulatory abnormalities. Diseases such as systemic lupus erythematosis, chronic rheumatoid arthritis, multiple sclerosis and psoriasis have in common the appearance of autoantibodies and self-reactive lymphocytes.
Multiple sclerosis is the most common neurological disease of young people. It is believed to cost more in medical care and lost income than any other neurological disease of young adults.
Multiple sclerosis affects the myelin sheaths of nerves. Myelin is an insulating material that coats most axons and allows rapid signal conduction over long distances by saltatory conduction. It is thought that antibodies and specialised cells of the immune system attack the myelin coating. This process leads to inflammation and scarring (sclerosis) which damages blood vessels in the area by the formation of a lesion known as a plaque. These plaques are characterised by being infiltrated by macrophages and T cells. This results in demyelination with the consequential loss of the rapid signal conduction.
A possible method of treating these autoimmune and inflammatory diseases is by suppressing T-cell proliferation and modulating their activation.
The early stages of T-cell activation may be conceptually separated into pre-Ca2+ and post-Ca2+ events (Cahalan and Chandy 1997, Curr. Opin. Biotechnol. 8 749). Following engagement of the T-cell receptor by an antigen, activation of tyrosine kinases and the generation of inositol 1,4,5-triphosphate lead to the influx of Ca2+ and a rise in the cytoplasmic Ca2+ concentration. The rise in Ca2+ activates the phosphatase calcineurin, which then dephosphorylates a cytoplasmically localized transcription factor (N-FAT) enabling it to accumulate in the nucleus and bind to a promoter element of the interleukin-2 gene. Along with parallel events involving the activation of protein kinase C and ras, gene transcription leads to lymphokine secretion and to lymphocyte proliferation. Some genes require long-lasting Ca2+ signals while others require only a transient rise of Ca2+.
Ion channels underlie the Ca2+ signal of T-lymphocytes. Ca2+ ions move across the plasma membrane through a channel termed the store-operated Ca2+ channel or the calcium release-activated Ca2+ channel. Two distinct types of potassium channels indirectly determine the driving force of calcium entry. The first is the voltage-gated Kv1.3 channel (Cahalan 1985, J. Physiol. 385: 197; Grissmer 1990, Proc. Natl. Acad. Sci. USA 87 9411; Verheugen 1995, J. Gen. Physiol. 105 765; Aiyar 1996, J. Biol. Chem. 271 31013; Cahalan and Chandy 1997, Curr. Opin. Biotechnol. 8 749) and the second is the intermediate-conductance calcium-activated potassium channel, IKCa1 (Grissmer 1993, J. Gen. Physiol. 102 601; Fanger 1999 J. Biol. Chem. 274 5746; Rauer 1999, J. Biol. Chem. 274 21885; VanDorpe 1998, J. Biol. Chem. 273 21542; Joiner 1997, Proc. Natl. Acad. Sci. USA 94 11013; Khanna 1999, J. Biol. Chem. 274 14838; Lodgson 1997, J. Biol. Chem. 272 32723; Ghanshani 1998, Genomics 51 160). When these potassium channels open, the resulting efflux of K+ hyperpolarizes the membrane, which in turn accentuates the entry of Ca2+, which is absolutely required for downstream activation events (Cahalan and Chandy 1997, Curr. Opin. Biotechnol. 8: 749).
The predominant voltage-gated channel in human T-lymphocytes is encoded by Kv1.3, a Shaker-related gene. Kv1.3 has been characterised extensively at the molecular and physiological level and plays a vital role in controlling T-lymphocyte proliferation, mainly by maintaining the resting membrane potential of resting T-lymphocytes. Inhibition of this channel depolarises the cell membrane sufficiently to decrease the influx of Ca2+ and thereby prevents downstream activation events.
Accordingly, compounds which are selective Kv1.3 blockers are potential therapeutic agents as immunosuppressants for the prevention of graft rejection, and the treatment of autoimmune and inflammatory disorders. They could be used alone or in conjunction with other immunosuppressants, such as selective IKCa1 blockers or cyclosporin, in order to achieve synergism and/or to reduce toxicity, especially of cyclosporin.
Developments in the field of voltage-gated K-channel electrophysiology have strengthened the case for treating of multiple sclerosis and also diabetes mellitus by inhibiting the Kv1.3 channel. It was found that autoreactive T-cells from multiple sclerosis patients exhibit highly elevated levels of Kv1.3 (Wulff, H et al (2003) J. Clin Invest. 111 (11) 1703-1713). ShK-K22Dap, a selective peptide blocker of Kv1.3, potently inhibited the proliferation of T-cells with this high-Kv1.3 phenotype. (Beeton, C. et al (2001) PNAS 98 13942-13947). The connection between T-cell replication and Kv1.3 blockade has also been shown through the use of a small molecule, a psoralen derivative, that is an active and relatively specific inhibitor of the Kv1.3 channel. The derivative showed specificity in inhibiting the proliferation of the high Kv1.3 T-cells over peripheral blood T-cells (Vennekamp et al (2004) Mol. Pharm. 65 1364-1374).
The Kv1.3 channel has also been associated with diabetes. Studies of Kv1.3 knockout mice found that the mice have increased insulin sensitivity. The selective blockage of the Kv1.3 channel also led to increased insulin sensitivity (Xu, J. et al. (2004) PNAS 101 (9), 3122-3117). It has been suggested by Wulff, who was involved in the electrophysiology on multiple sclerosis that diabetes also involves autoreactive T-cells that express very high levels of Kv1.3 (Wulff, H. et al. (2003) Curr. Op. DDD. 6 640-647).
At present there exist a number of non-selective potassium channel blockers that will inhibit lymphocyte proliferation, but have adverse side effects. Other potassium channels exist in a wide range of tissues including the heart and brain, and generally blocking these channels is undesirable. Accordingly it would be advantageous to provide or identify compounds, which are selective inhibitors of the Kv1.3 channel.
U.S. Pat. No. 5,494,895 discloses the use of a thirty-nine amino acid peptide, scorpion peptide margatoxin, as a selective inhibitor and probe of Kv1.3 channels present in human lymphocytes, and also as an immunosuppressant. However the use of this compound is limited by its potent toxicity.
International Patent Application publication No's WO 97/16438 and WO 09/716,437, and U.S. Pat. No. 6,051,590 describe the use of the triterpene, correolide and related compounds as immunosuppressants. The potential for these compounds to become immunosuppressants was illustrated by experiments showing their attenuation of the delayed-type hypersensitivity (DTH) response in mini-swine.
U.S. Pat. No. 6,077,680 describes DNA segments and proteins derived from sea anemone species, more particularly ShK toxin from Stichodactyla helianthus. The ShK toxin was found to block Kv1.1, Kv1.3, Kv1.4 and Kv1.6, but a mutant ShK-K22DAP was found to selectively block Kv1.3. Unfortunately the mutant did not exhibit the requisite pharmacokinetic profile for clinical use. A recently reported ShK analog, ShK(L5), was at least 100-fold more active against Kv1.3 (Kd=69 pM) than Kv1.1 and furthermore it showed at least 250-fold selectivity over every other relevant member of the Kv1 family (Beeton et al. (2005) Mol. Pharm. 67:1369-1381).
Both ShK toxin and ShK(L5) were shown to both prevent and treat experimental autoimmune encephalomyelitis in Lewis rats, an animal model for human multiple sclerosis (Beeton, et al. (2001) Proc. Natl. Acad. Sci. USA 98 13942), by selectively targeting T-cells chronically activated by the myelin antigen, MBP (myelin basic protein). The same study also indicated that chronically activated encephalitogenic rat T-cells express a unique channel phenotype characterised by high expression of Kv1.3 channels (approximately 1500 per cell) and low numbers of IKCa1 channels (approximately 120 per cell). This channel phenotype is distinct from that seen in quiescent and acutely activated cells and may be a functionally relevant marker for chronically activated rat T lymphocytes.
Other compounds which are blockers of Kv1.3 include psoralens (Vennekamp et al. (2004) Mol. Pharm. 65, 1365-1374 and Wulff at al., US 2006/0079535) and selected benzamides (Schalhofer et al. (2002) Biochem. 41, 7781-7794 and Schalhofer et al (2003) Biochem. 42, 4733-4743). The reported compounds in these classes did not exhibit drug-like characteristics.
Khellinone, a substituted benzofuran and natural product from certain plants, and 8-Methoxypsoralen (8-MOP), both commercially available products, have been found to exhibit weak blocking activity on the Kv1.3 channel.

The present invention is directed to a new class of compounds which exhibit useful Kv1.3 channel blocking activity.